It is important to prevent generation of rust on a rolling bearing which is used in an electric auxiliary machine for use in a car to or into which water on a road, salt water or seawater is liable to attach or penetrate during travel of the car and on a rolling bearing which is used in iron-manufacturing equipment such as a rolling roller always subjected to penetration of cooling water.
A car travels on the seashore and a road on which calcium chloride or the like is scattered to prevent the surface of the road from being frozen. Thus the use of the car has diversified and grease is increasingly demanded to have a high rust resisting performance.
Further owing to the transition of molding powder which is used in a successive casting equipment of an iron-manufacturing industry, a part of the components of the molding powder dissolves in cooling water. Thereby the generation of rust is accelerated. Consequently grease is increasingly demanded to have a high rust. resisting performance.
To restrain the generation of rust, the rolling bearing is appropriately sealed or a mechanical measure is taken to thereby prevent the penetration of substances causing the generation of rust. For example, in a car, a measure of disposing an alternator or the like at a position where the rolling bearing is not splashed with muddy water and a measure of mounting a mudguard thereon are taken. But it is impossible to completely seal the rolling bearing owing to its mechanism. Thus the lubricating grease to be enclosed in the rolling bearing is demanded to have an excellent rust resisting performance.
Conventionally, as rust resisting grease excellent in its rust resisting performance, an oil-soluble inhibitor serving as a rust resisting agent, a water-soluble inorganic passivating agent, and a grease composition containing a nonionic surface active agent are known (see patent document 1).
As a grease-enclosed rolling bearing for use in an alternator, a grease composition containing a passivating oxidizing agent, organic sulfonate, and polyvalent alcohol ester is known (see patent document 2).
It is known that sodium nitrite excellent as a water-soluble inorganic passivating agent reacts with secondary amine in an acidic condition to generate nitrosamine. The nitrosamine is a substance of applying a load to environment. The following grease composition to be enclosed in a bearing is known. The grease composition contains 0.1 to 10 parts by weight of each based on the total amount of the grease composition of a lipophilic organic inhibitor, a nonionic surface active agent, and a hydrophilic organic inhibitor selected from among a lanolin fatty acid derivative and an alkanolamine derivative modified with a hydrophilic radical; and barium sulfonate serving as a rust resisting agent (see patent document 3).
In addition, as an additive not containing nitrite which is the substance of applying a load to environment, a grease composition containing ester of polyvalent alcohol and synthetic hydrocarbon oil as the base oil thereof is known (see patent document 4).
But there is a fear that the grease containing the sodium nitrite adversely affects the environment of the earth. When a demand for a high rust resisting performance becomes strong, there is a case in which the grease composition containing the ester of the polyvalent alcohol and the synthetic hydrocarbon oil as the base oil thereof has an insufficient rust resisting performance.
Patent document 1: Japanese Patent Application Laid-Open No. 3-200898
Patent document 2: Japanese Patent Application Laid-Open No. 5-263091
Patent document 3: Japanese Patent Application Laid-Open No. 11-279578
Patent document 4: Japanese Patent Application Laid-Open No. 2004-51912